I have just tried Kingston 4 GB mini SDHC Class4 memory card , it doesn't work. If you are planing to buy it , choose another SD card .

Have you tried putting this card potected/unprotected? Even my camera not always recognizes Kingston's SD cards, but I thought the one you mentioned is officialy supported. I had problem earlier with Kingston's 2 GB mini (put through adapter), but after several tries it started to work and now it's recognized everytime (strange behavior).

Yesterday i tried the SD card in 2 different Zens. I gave up, i slept and i woke up. In the morning ,i turned it on, it worked.
It seems like Zen is the type which pretends it wont go all the way at first date but changes mind at night.

And that's really important information, Creative doesn't mention any 16GB SDHC cards on their support list.

Yes, even though they cant physically test every card on the market, Im sure there will be many more that will work, but wont be listed as tested on Creatives site. It is SDHC compatible so theoretically it should support the soon to be released 32GB SDHC cards as well. My 16GB A Data card is the one from Newegg by the way.

Class 6 means sustained write speed of at least 6 MB/s on an empty card. The speed will also depend on the writing speed of the card slot. Expansion slots with slower transfer rates will prevent the card from reaching its highest sustained write speed.

The 16GB A-data SDHC card works great on my Cowon D2. It's SDHC compatible, so it should work fine with the Zen.

Edit: see BobbyRS's post above. According to him, 16 GB A-data SDHC cards work with the Zen.

Take a look at this post from Slydini. It directs you to a Zen compatible card list (from Creative), and they mention a Transcend 8 GB Class 6 SDHC card. Compare the part number with the one of the card on Amazon.com.